1. Field of the Invention
The present invention relates to a tube for transporting fuels, and more particularly to a tube for use as a fuel tube in transportation vehicles such as automobiles. The tube according to the present invention has excellent barriering properties (barrier action) against not only a gasoline-based fuel, but also a mixed fuel including alcohols, and is kink-proof to an extent that it can be used in a bent state in a relatively small space.
2. Description of the Related Art
Tubes having a multilayer structure in which a metal pipe, a rubber hose, a single layer tube of nylon, and various resins are layered have conventionally been used as tubes for transporting fuels in a transportation vehicle such as an automobile.
On the other hand, the realization that it may be difficult to secure an ample supply of gasoline in the future has led to research for substitute fuels in the place of gasoline. As one such substitute fuel, mixed fuels in which alcohols, such as methanol, are added to gasoline have been studied, and have already been put to practical use in the West. Use of methanol as a substitute fuel has attracted much attention because it is expected to provide an improved octane value and contribute to the increased cleanness of the exhaust gas.
In particular, in view of the current state of the atmospheric environment, it is desirable to minimize the amount of fuel to be discharged from a transportation vehicle. This has led to the enforcement of increasingly strict emission regulations. Therefore, tubes are desired which hardly permit fuels to escape therethrough.
Fuel tubes are often incorporated inside the housing so as to prevent them from being damaged from the impact of a collision and to protect them against flying stones, fire, etc. originating from the outside. In such cases, it is particularly required to substantially eliminate the transmission of gas through the fuel tube because the odor of the transmitted gas might cause discomfort to people in the vehicle, or might even catch fire when the concentration thereof becomes sufficiently high.
On the other hand, automobiles allow less and less space for tubes to be incorporated, so that it is required that the tubes be provided in such a manner as to avoid other equipment in the engine housing.
Moreover, as is well known, transportation vehicles are required to have improved resistance against rust and a reduced mass in view of the demands for increased durability and improved fuel consumption.
A conventional tube for fuel transportation is disclosed in Japanese Laid-Open Patent Publication No. 4-224384. This tube for fuel transportation is a single layer tube composed of a polyester type resin or alternatively a multilayer tube whose innermost layer is composed of a polyester type resin. In this tube for fuel transportation, polybutylene terephthalate, which has excellent barrier action, is used as the polyester type resin so as to improve the barrier action of the tube.
German Patent Application Publication Nos. 4112662, 4137430, 4137431, and 4215608 each disclose a conventional tube for fuel transportation having a multilayer structure including an inner layer and an outer layer composed essentially of polyamide and a middle layer composed of linear crystalline polyester.
In the above-mentioned Patent Application Publications, it is proposed to form the middle layer of a mixture of linear crystalline polyester and polyamides or compounds having various reactive groups in order to improve the adhesion between the respective layers.
However, the inventors of the present invention have confirmed the following drawbacks of a tube for fuel transportation having a multilayer structure including an innermost layer composed of polybutylene terephthalate:
i) There is a limit to the improvement of barrier action against the fuel. Specifically, a fuel for a transportation vehicle, such as an automobile, is generally a mixture of various components. Therefore, a tube may show excellent barrier action for a fuel of one composition but not for a fuel of another composition, thus resulting in insufficient overall barrier action for fuels. PA1 ii) A tube disclosed in an example of the above-mentioned Japanese Laid-Open Patent Publication No. 4-224384, in which the inner layer is composed of polybutylene terephthalate and the outer layer is composed of nylon 12, has poor adhesion between the inner and outer layers, thus resulting in poor kink-proofness. Tubes for fuel transportation of this kind, used for transportation vehicles, are likely to be used in a very small space and bent in a circle with a small radius. Therefore, these tubes are required to be highly kink-proof in order to have practicality. PA1 iii) In cases where a tube for fuel transportation whose innermost layer is composed of a polyester type resin such as polybutylene terephthalate is used for transportation of a mixed fuel including alcohols, the innermost layer may deteriorate through hydrolysis caused by a small amount of moisture contained in the alcohols. Such a tube has the problem of poor durability.
Moreover, an experiment conducted by the inventors of the present invention has revealed the following drawback of a tube for fuel transportation having a multilayer structure including a middle layer composed of a mixture of polybutylene terephthalate and various compounds in order to realize improved adhesion with the polyamide constituting the inner and outer layers: There is a limit to the improvement of the barrier action against fuels. Specifically, as described later with reference to the Comparative Examples, a tube including a middle layer composed of a linear crystalline polyester which is either polybutylene terephthalate or a mixture obtained by mixing polybutylene terephthalate with a polyamide resin, a maleic acid anhydride modified ELM, an ethylene-ethylacrylate-glycidyl methacrylate copolymer, or the like has poor barrier action against fuels although the middle layer is formed of polybutylene terephthalate.